1. Field of the Invention
This invention relates to a method for detecting an unexposed region of a recording medium, wherein a judgment as to whether the overall region or a partial region of the recording medium has or has not been exposed is based on the values of certain components of an image signal detected from the recording medium, which may have a radiation image stored thereon.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen for the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor, which has been exposed to the radiation, is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 55(1980)-163472, 56(1981)-104645, and 55(1980)-116340, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation, which has passed through an object such as the human body, in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during the exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and the image signal is used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light emitted by the stimulable phosphor varies over a wide range and is proportional to the amount of energy stored during its exposure to the radiation, it is possible to obtain a visible image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain a desirable image density, an appropriate read-out gain is set when the emitted light is being detected with a photoelectric read-out means and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device such as a CRT.
In order to detect an image signal accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet and the like. A novel radiation image recording and reproducing system which accurately detects an image signal has been proposed in, for example, Japanese Unexamined Patent Publication Nos. 58(1983)-67240, 58(1983)-67241 and 58(1983)-67242. The proposed radiation image recording and reproducing system is constituted such that a preliminary read-out operation (hereinafter simply referred to as the "preliminary readout") is carried out in order approximately to ascertain the radiation image stored on the stimulable phosphor sheet. In the preliminary readout, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as the "final readout") is carried out to obtain the image signal, which is to be used during the reproduction of a visible image. In the final readout, the stimulable phosphor sheet is scanned with a light beam having an energy level higher than the energy level of the light beam used in the preliminary readout, and the radiation image is read out with the factors affecting the image signal adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal. The term "read-out conditions" as used hereinafter means a group of various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image readout and the output of a read-out means. For example, the term "read-out conditions" may refer to a read-out gain and a scale factor, which define the relationship between the input to the read-out means and the output therefrom, or to the power of the stimulating rays used when the radiation image is read out.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the irradiated light beam, i.e. the sensitivity of the stimulable phosphor sheet to the irradiated light beam depends upon the wavelength of the irradiated light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed at which the stimulable phosphor sheet is scanned with the light beam may be changed. Regardless of whether the preliminary readout is or is not carried out, it has also been proposed to analyze the image signal (including the preliminary read-out image signal) obtained and to adjust image processing conditions, which are to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets.
In the aforesaid radiation image recording and reproducing systems, a subdivision image recording operation is often carried out wherein the whole recording area of a single recording medium (such as a stimulable phosphor sheet or X-ray film) is divided into a plurality of regions and different radiation images are recorded in each region. In such cases, for example, the subdivision pattern is found from the preliminary read-out image signal, and appropriate read-out conditions are calculated for each of the subdivided regions. A final readout is then carried out under conditions set according to the calculations thus performed. However, in a case where, for example, the whole recording area of a recording medium was divided into four regions and radiation images were recorded in only three of the four regions with the remaining region remaining unexposed to radiation, the image signal obtained from an image read-out operation performed on the recording medium includes image signal components corresponding to the unexposed region. In such cases, if the read-out conditions for the final readout are calculated in the same manner as that described above, the read-out conditions, which are ultimately set for the final readout, will be set incorrectly. This is because the ultimate read-out conditions for the final readout are determined from image signal components corresponding to both exposed and unexposed regions. Therefore, during the final readout, image signal components having inappropriate values are obtained from regions in which radiation images were actually recorded.
Besides cases where the whole recording area of a recording medium is divided into a plurality of regions and a plurality of radiation images are recorded in the subdivided regions, a recording medium which has not been exposed to radiation may accidently be put in a read-out apparatus in order that an image read-out operation may be carried out on the recording medium. In such cases, when the read-out conditions for the final readout are calculated from the preliminary read-out image signal obtained during the preliminary readout, the calculated read-out conditions are markedly different from normal read-out conditions. Therefore, if the read-out conditions for the final readout are set according to the calculations, the voltage applied to a photomultiplier serving as a read-out means, for example, or the amplification factor of an amplifier will be made markedly large. As a result, various adverse effects arise. For example, the performance of parts of the read-out apparatus may deteriorate.